Considering how many people just share a screenshot of the NK Archive game menu, maybe some people don't know how to take screen shots of the actual game they're playing, but the rest are definitely capturing the menu for Lols.

If you can't take screenshots at all, then make sure the Steam Settings -> In Game -> Steam Overlay is Enabled, and that you're using the right button for screenshots (F12 by default).

With NK Archive, there's two windows -- The game selection menu, and the game itself. The selection menu is what Steam thinks you want to take a screen shot of, even when the game window is full screened. Convince Steam that the menu isn't exciting by minimising that window.

With the NK Archive window minimized, feel free to share the epic game moment you achieved.